Conventionally, some kind of matting system is used to relieve standing stress and fatigue or to attenuate falling impact stress. Many such systems are known. Some use closed-cell foam or vulcanized rubber chips bound together to provide a shock absorbing structure. Others use an array of deformable elastomeric cells under an elastomeric surface layer. The problems with most conventional matting systems are twofold. Foam and most other conventional impact attenuating structures actually get harder as the force applied to them increases, and they can bottom out with hip, elbow or head impacts, or even just prolonged standing. In most conventional systems, the force absorbing mechanism begins with an immediate displacement at the very surface of the mat, a displacement that often leads to dangerous foot entrapment and/or other displacement instability for those who play, walk or work on such a surface.
It is generally acknowledged that standing for long periods induces discomfort and fatigue, especially in the lower body. A high proportion (>80%) of industrial workers required to stand for long periods report lower leg or foot discomfort. It is also generally agreed and documented in several scientific studies that standing on a soft surface improves perceptions of comfort and reduces perceptions of fatigue or “tiredness”.
The results of research aimed at documenting the physiological basis for antifatigue mats are more equivocal. Standing for long periods often leads to edema (swelling) of the lower legs and feet and antifatigue mats can provide relief. The conventional view is that soft surfaces cause more adjustments to posture, activating the venous pumping that returns blood to the heart, thus reducing swelling and discomfort.
A recent study by Madeleine et al (1998), “Subjective, physiological and biomechanical responses to prolonged manual work performed standing on hard and soft surfaces”, Eur. J. Appl. Physiol. 77:1-9, however compares responses from people that work while standing for two hours on either hard surface or a soft antifatigue mat. The data reported in this study conflict somewhat with the conventional wisdom.
As to the need for impact safety matting, falls onto hard surfaces are a significant cause of injury and accidental death, especially among the elderly. Among non-fatal injuries due to falling, hip fractures are the most common and the most severe. The loss of mobility following a hip fracture is itself a potentially fatal risk and many elderly patients never return to normal activity after a fall. Given the significant human and medical cost of high hip fracture rates, researchers have explored ways of reducing the rate of hip fractures among the elderly. Proposed strategies include the use of protective hip pads, cushioned flooring and the promotion of exercise programs to increase the strength and agility of at-risk individuals. We review the injury reduction potential of cushioned floors, and calculations as to the effect of compliant flooring materials on the peak impact force acting at the hip.
The fracture strength of the femoral head has been estimated using mechanical tests of cadaveric specimens, finite element modeling and predictions based on material properties. From such studies, the peak lateral loads inducing fractures in older individuals range from 1000 to 6000 N. Younger subjects have greater femoral strength. Fracture strength depends on many factors, including the loading conditions and the age, body size and bone mineral density of the subject.
The force acting at the hip during a fall is affected by a number of factors, most notably the impact velocity, the effective mass involved in the impact, the material properties of the soft tissue overlying the hip and the properties of the surface against which the impact occurs. A group of researchers from Harvard University and Harvard Medical School (Robinovitch et al, 1991) used a constrained release experiment to determine the non-linear stiffness and damping properties of soft tissue and used their results to calculate the impact force on a hard surface. The predicted impact force magnitudes were similar to the breaking strength of the femoral neck, supporting the idea that unprotected falls onto hard surfaces can break the hip. The Harvard hip impact model uses non-linear stiffness and damping functions to describe the viscoelastic properties of the soft tissue overlying the hip and documents the soft tissue parameters for males and female subjects across a range of soft tissue thicknesses.
FIG. 22 is a schematic of the Harvard hip impact model, in which impact of the falling mass m is moderated by the compliant material properties of the soft tissue. Soft tissue behavior is characterized by non-linear stiffness (kt) and damping (ct) functions.
Equation of Motion
The response of the system in FIG. 22 is described by a differential equation in xm:F=m(d2xm/dt2)=ct(dxm/dt)=ktxm  (1)
In order to calculate the peak force of an impact, the model requires parameters for soft tissue properties, the effective mass and a description of the initial conditions (e.g. impact velocity) defining the impact.
Soft Tissue Properties:
Robinovitch (1991), gives non-linear functions for kt and ct for both males and females and for different muscle activation states. For the purposes of the analysis presented here, values for male subjects in a muscle-relaxed state were used. Specifically,kt=90,440(1−e−F/114)  (2)ct=756(1−e−F/108)  (3)Effective Mass:
Robinovitch (1991) reports the average effective mass (m) involved in hip impact to be 39 kg for males in the muscle-relaxed state.m=39.0  (4)Initial conditions:
Van den Kroonenberg et al (1993) report estimated hip-floor impact velocities ranging from 2.14 to 4.25 m s−1 and averaging 3.19 m s−1.dxm/dt(t=0)=3.19 ms−1  (5)    Also at time t=0:    xm=0    xs=0    d2xm/dt2=g=9.81 ms−2 